1) For uptake measurements, grow yeast cells to logarithmic phase in YNB medium supplemented with 3% glucose and 1 mM L-arginine.
2) Harvest cells at OD₆₀₀ of 0.6 to 0.8, wash, and resuspend in 40 mM potassium phosphate buffer, pH 7, to a final OD₆₀₀ of 8.
3) Five minutes before the uptake measurement, supplement cells with 6% glucose and incubate at 30ºC.
4) To start the reaction, add 450 µL of cell suspension to 400 µL of the same buffer containing 200 µM methylammonium and 10 nM ¹⁴C-methylammonium (2.11 Gbq/mmol; Amersham).
5) After 0.5, 1, 2, and 4 min, withdraw 200 µL aliquots, dilute in 5 mL ice-cold potassium phosphate buffer containing 200 mM methylammonium, and filter through glass fiber filters (GF/C; Whatman).
6) Wash filters twice with 5 mL of cold deionized water and analyze by liquid scintillation spectrometry.